It is well known for lubricating oils to contain a number of surface active additives (including antiwear agents, dispersants, or detergents) used to protect internal combustion engines from corrosion, wear, soot deposits and acid build up. Often, such surface active additives can have harmful effects on engine component wear (in both iron and aluminium based components), bearing corrosion or fuel economy. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). It is believed that ZDDP antiwear additives protect the engine by forming a protective film on metal surfaces. ZDDP may also have a detrimental impact on fuel economy and efficiency and copper corrosion. Consequently, engine lubricants may also contain a friction modifier to obviate the detrimental impact of ZDDP on fuel economy and corrosion inhibitors to obviate the detrimental impact of ZDDP on copper corrosion. Other additives may also increase lead corrosion.
Developments in engine design have resulted in engines that employ iron-containing and/or non-ferric components. Typically non-ferric engine components thereof are based on aluminium-alloy, silicates, oxides, or other ceramic materials. Antiwear additives such as ZDDP is believed to result in poorer engine wear performance in aluminium-alloy based engine compared with ferric based engines.
Further, engine lubricants containing phosphorus compounds and sulphur have been shown to contribute in part to particulate emissions and emissions of other pollutants. In addition, sulphur and phosphorus tend to poison the catalysts used in catalytic converters, resulting in a reduction in performance of said catalysts.
In light of the move to more strict emissions standards and the deleterious effects of certain elements, there is a desire for reduced amounts of sulphur, phosphorus and sulphated ash in engine oils. Consequently, the amounts of phosphorus-containing antiwear agents such as ZDDP, overbased detergents such as calcium or magnesium sulphonates and phenates have been reduced. As a consequence, ashless additives such as esters of polyhydric alcohols or hydroxyl containing acids including glycerol monooleate have been contemplated to provide friction performance.
Canadian Patent CA 1 183 125 (by Barrer, filed Sep. 10, 1981) discloses lubricants for gasoline engines containing alkyl-ester tartrates, where the sum of carbon atoms on the alkyl groups is at least 8. The tartrates are disclosed as antiwear agents. Other references disclosing tartrates and/or tartrimides include International Publication WO 2006/044411, and US Patent Applications for internal combustion engines requiring reduced amounts of sulphur, sulphated ash, and phosphorus. The lubricant composition has anti-wear or anti-fatigue properties. The lubricating compositions are suitable for road vehicles.
U.S. Pat. No. 4,237,022 (by Barrer, filed Dec. 2, 1980) discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 5,338,470 (by Hiebert, filed Dec. 10, 1992) and International Publication WO 2005/087904 (by Migdal, filed Mar. 11, 2004) disclose lubricants containing at least one hydroxycarboxylic acid ester or hydroxy polycarboxylic acid (in particular citrates or ethyl glycolate). The lubricant composition has anti-wear or anti-fatigue properties.
International Application WO2008/070307 (by Brown, filed Oct. 22, 2007) discloses engine lubricants containing antiwear agents based on malonate esters.